FIG. 1 illustrates a prior art arrangement for measuring the amount of radio frequency (RF) power transmitted from an RF power amplifier (PA) 10, to an antenna 12 through a transmission line 13. A directional coupler 14 diverts a small portion of the power to a detector 16 which measures the power transferred in the forward direction.
In some applications, both the forward and reverse (reflected) power must be measured simultaneously, for example, to determine the voltage standing wave ratio (VSWR). FIG. 2 illustrates how two directional couplers 14 and 18 may be arranged in series. One directional coupler 14 enables the first detector 16 to measure the forward power, while the other directional coupler 18 enables a second detector 20 to measure the reverse power.
Directional couplers tend to have low insertion loss, i.e., the loss in power resulting from the insertion of the coupler into the transmission line, because they have low series resistance. The insertion loss of a typical directional coupler may be about 0.1-0.2 dB. However, they also tend to be narrowband devices which restricts their use to applications where the signals to be measured fall within a relatively narrow frequency range, for example, from several hundred megahertz (MHz) to a few gigahertz (GHz).
FIG. 3 illustrates a prior art arrangement in which a directional bridge 22 is used in place of a coupler. Directional bridges are resistor bridges that are designed to divert a portion of the transmitted signal traveling in one direction to a detector. Because they are fabricated from resistive materials, they are inherently broadband devices that can operate over very wide frequency ranges, for example, from essentially DC to tens of GHz where the upper end is limited by parasitics. The resistive nature of a directional bridge, however, typically results in relatively high insertion loss, for example, 1.0-2.0 dB. Although the insertion loss of a single directional bridge is typically tolerable, if power must be measured in both directions, the use of two opposing, series-connected directional bridges introduces an unacceptable level of insertion loss.